1. Field of the Invention
The present invention relates to a technique for plating semiconductor wafers, and more particularly to a cup-type plating apparatus.
2. Description of the Related Art
Conventionally, a cup-type plating apparatus is a known apparatus for plating semiconductor wafers. In the cup-type plating apparatus, a wafer is placed on an upper end portion of a plating tank, and a plating solution is fed in the form of an ascending flow from a lower portion of the plating tank toward a surface-to-be-plated of the wafer (hereinafter referred simply as a "wafer surface") to thereby plate the wafer surface. Being suitable for automation of a small-lot production or plating process, the cup-type plating apparatus is in wide use.
However, the cup-type plating apparatus involves the following two problems. First, film, such as black film, formed on the surface of an anode during plating exfoliates to become impurities in the plating solution. The impurities are carried in the ascending flow of the plating solution and reach the wafer surface, causing nonuniform plating.
Second, in the case where an insoluble anode is used, an additive added for control of plating performance is consumed in significant amounts. The reason is that use of an insoluble anode causes decomposition of the additive present in the vicinity of the insoluble anode although dissolution of an anode metal into the plating solution does not occur. Such an additive-consuming phenomenon not only complicates plating process control but also increases plating cost.
A conceivable solution to the above two problems is installation of a diaphragm within the plating tank; that is, partitioning of the interior of the plating tank into an anode-side chamber and a wafer-side chamber. Employment of such a diaphragm is disclosed in, for example, Japanese Utility Model Application Laid-Open (kokai) No. 36529/1987 and Japanese Patent Application Laid-Open (kokai) Nos. 242797/1989 and 154989/1992.
According to Japanese Utility Model Application Laid-Open (kokai) No. 36529/1987 and Japanese Patent Application Laid-Open (kokai) No. 242797/1989, the diaphragm is disposed within the plating tank and above an anode in such a manner as to cover the entire surface of the anode. This diaphragm can prevent mixing of impurities generated from the anode in a plating solution that ascends toward a wafer surface. Also, since the anode is separated from the wafer surface, the consumption of additive can be reduced in the case where an insoluble anode is used. However, in the case where the plating solution is fed at a certain position, the thus-arranged diaphragm may interrupt a direct ascending flow of a plating solution. As a result, the plating solution fails to flow smoothly. Further, bubbles and impurities generated from the anode stagnate under the diaphragm, which is disposed horizontally within the plating tank, thereby preventing stable supply of plating current.
According to Japanese Patent Application Laid-Open (kokai) No. 154989/1992, the interior of a plating tank is partitioned into an upper wafer-side chamber and a lower anode-side chamber by means of a diaphragm. These chambers are fed with a plating solution separately from each other. Through feed of the plating solution into the anode-side chamber, bubbles and impurities flow with the flowing plating solution and thus are less likely to stagnate under the diaphragm. Nevertheless, impurities and bubbles are apt to stagnate under the horizontally disposed diaphragm. Although the plating solution is fed into the upper and lower chambers separately from each other, the plating solution flowing out from the upper chamber and that flowing out from the lower chamber are mixed in a single plating solution storage tank. As a result, the impurity content of the plating solution increases, which has an adverse effect on plating.